Stents are widely used for supporting a lumen structure in a patient's body. For example, stents may be used to maintain patency of a coronary artery, carotid artery, cerebral artery, femoral artery, other blood vessels including veins, or other body lumens such as the ureter, urethra, bronchus, esophagus, or other passage.
Stents are commonly metallic tubular structures made from stainless steel, Nitinol, Elgiloy, cobalt chrome alloys, tantalum, and other metals, although polymer stents are known. Stents can be permanent enduring implants, or can be bioabsorbable at least in part. Bioabsorbable stents can be polymeric, bio-polymeric, ceramic, bio-ceramic, or metallic, and may elute over time substances such as drugs. Non-bioabsorbable stents may also release drugs over time. Stents are passed through a body lumen in a collapsed state. At the point of an obstruction or other deployment site in the body lumen, the stent is expanded to an expanded diameter to support the lumen at the deployment site.
In certain designs, stents are comprised of tubes having multiple through holes or cells that are expanded by inflatable balloons at the deployment site. This type of stent is often referred to as a “balloon expandable” stent. Stent delivery systems for balloon expandable stents are typically comprised of an inflatable balloon mounted on a two lumen tube. The stent delivery system with stent compressed thereon can be advanced to a treatment site over a guidewire, and the balloon inflated to expand and deploy the stent.
Other stents are so-called “self expanding” stents and do not use balloons to cause the expansion of the stent. An example of a self-expanding stent is a tube (e.g., a coil of wire or a tube comprised of cells) made of an elastically deformable material (e.g., a superelastic material such a nitinol). Some self expanding stents are also comprised of tubes having multiple through holes or cells. This type of stent is secured in compression in a collapsed state to a stent delivery device. At the deployment site, stent compression is released and restoring forces within the stent cause the stent to self-expand to its enlarged diameter.
Other self-expanding stents are made of so-called shape-memory metals. Such shape-memory stents experience a phase change at the elevated temperature of the human body. The phase change results in expansion from a collapsed state to an enlarged state.
A very popular type of self expanding stent is a cellular tube made from self-expanding nitinol, for example, the EverFlex stent from ev3, Inc. of Plymouth, Minn. Cellular stents are commonly made by laser cutting of tubes, or cutting patterns into sheets followed by or preceded by welding the sheet into a tube shape, and other methods. Another delivery technique for a self expanding stent is to mount the collapsed stent on a distal end of a stent delivery system. Such a system can be comprised of an outer tubular member and an inner tubular member. The inner and outer tubular members are axially slideable relative to one another. The stent (in the collapsed state) is mounted surrounding the inner tubular member at its distal end. The outer tubular member (also called the outer sheath) surrounds the stent at the distal end.
Prior to advancing the stent delivery system through the body lumen, a guide wire is first passed through the body lumen to the deployment site. The inner tube of the delivery system is hollow throughout at least a portion of its length such that it can be advanced over the guide wire to the deployment site. The combined structure (i.e., stent mounted on stent delivery system) is passed through the patient's lumen until the distal end of the delivery system arrives at the deployment site within the body lumen. The deployment system and/or the stent may include radiopaque markers to permit a physician to visualize positioning of the stent under fluoroscopy prior to deployment. At the deployment site, the outer sheath is retracted to expose the stent. The exposed stent is free to self-expand within the body lumen. Following expansion of the stent, the inner tube is free to pass through the stent such that the delivery system can be removed through the body lumen leaving the stent in place at the deployment site.
In prior art devices, high forces may be required to retract the outer sheath so as to permit the stent to self expand. Delivery systems designed to withstand high retraction forces can be bulky, can have reduced flexibility and can have unacceptable failure rates. In addition, due to frictional forces between the stent and the outer sheath in prior art devices the stent may change in length during deployment, either in overall length or locally over regions of the stent. For example, long stents, thin stents, stents with high axial flexibility parallel to the central axis of the stent, or stents with a large amount of expansile force, when compressed in a sheath, tend to change in length as the outer sheath is withdrawn from the inner tubular member. Also, prior art delivery systems can be moved when the implant is partially deployed, resulting in undesirable regional length changes in the implanted device. Changes in stent length during stent deployment can prevent a stent from being properly deployed over the intended treatment area, can compromise stent fracture resistance and can compromise stent fatigue life.
What is needed is a stent delivery system that permits low force and precise delivery of stents without altering the intended length of the stent.